1. Field of the Invention
The present invention relates generally to chart recording devices and more particularly to devices which are suitable for producing rectilinear motion of the recording instrument transverse to the direction of travel of the chart medium.
2. Description of the Prior Art
One of the recording channels in a polygraph machine records galvanic skin resistance which is quite sensitive to subject responses due to the fact that galvanic skin resistance ("GSR") readings may vary widely during a typical polygraph test. This creates a need for a wide swinging pen movement in order to accurately and continuously measure this variable. Since a polygraph machine will typically also simultaneously record other body functions on separate recording pens, it is necessary that the recording pen which records galvanic skin resistance not collide with the other recording pens. Thus, the need to recenter the wide swinging GSR recording pen is decreased and a more homogeneous or unbroken recording is thereby produced.
Previous devices for recording galvanic skin resistance have used recording pens which track along a curvilinear or radial arc in response to the electrical signal being recorded. Curvilinear pen movement is, however, undesirable for several reasons. For example, in order to allow the recording pen to produce a tracing which includes the full width of the recording medium, the recording pen is typically placed so that at zero deflection it is far out beyond the tips of the other pens. While this design reduces the chances of the GSR pen from colliding with the other pens at extreme angular deflection, it also creates unnecessary errors in chart timing between it and the other recording pens. While this error can be reduced by lengthening the recording pen, the need for a compact design renders this design choice unacceptable. Furthermore, since it is the length of the arc which is proportional to the electrical signal being measured, interpretation of data is more difficult than would otherwise be the case if the recording was rectilinear. It would, therefore, be an improvement to provide a pen movement for recording galvanic skin resistance in a polygraph machine which pen movement produces a rectilinear tracing across the full width of the chart paper, yet which does not collide with the other pens and maintains a constant clearance or time difference with respect to the other pen recordings.
One device, to which the present invention is an improvement, is U.S. Pat. No. 3,206,756 to Thayer et al. which discloses means for converting a rotary motion into a rectilinear movement in a recording instrument. Rectilinear movement is accomplished in this device by a pen arm attached to a pulley which is in turn connected by a belt to a stationary drum. The pulley and the drum are also connected by a driving arm which is secured to a shaft which is coaxial with the drum and allows the pulley to rotate about the drum. Attachment tabs securing the belt to the pulley and drum are spaced radially outwards from the surface of the pulley and drum to prevent slippage
The following list of patent references discloses other devices which may have some general relevance to the subject invention:
______________________________________ U.S. Pat. No. Inventor's Name ______________________________________ 3,065,468 Edstrom 3,185,990 Millar 3,205,504 Nishimura et al. 3,754,281 Kampf 4,180,823 Guyot et al. 2,149,216 Gravley ______________________________________
U.S. Pat. No. 3,065,468 to Edstrom discloses a slave recording apparatus which is adapted to operate as a slave to a conventional rotating drum and pen type recorder, or in the alternative as a direct recorder. The device operates by driving a reciprocating table through a rack and pinion gear train by either an X axis or a Y axis drive mechanism. The X axis or Y axis drive mechanism which does not drive the rack and pinion gear train is linked to drive at least one scribe mounted independently of but adjacent to the reciprocating table in such manner as to be moveable in the same plane as a perpendicular to the direction of movement of the table.
U.S. Pat. No. 3,185,990 to Millar discloses a system for translating the angular motions of a galvanometer shaft into traces which are substantially straight or rectilinear and proportioned in relation to the angular motions of the galvanometer. The device includes a pair of pivotally mounted levers having their free ends converging towards each other and a link pivotally interconnecting the free ends of the levers. A stylus arm is connected with the link to move therewith in its pivotal motion and carries a recording tool cooperating with a recording element and located at the side of a shaft which oscillates about its axis in accordance with a variable to be recorded.
U.S. Pat. No. 3,205,504 to Nishimura et al. discloses a multi-point marking device for recorders wherein a pen carriage receiving a plurality of pens travels on a rotatable guide bar in response to the movement of a pulley system consisting of three sets of pulleys.
U.S. Pat. No. 3,754,281 to Kampf discloses a rectilinear stylus system for use with chart recorders. The stylus system comprises a stylus arm pivotally connected to a drive means which includes an endless flexible chain driven by a servomotor-operated sprocket. The stylus is secured at its free end for movement along a rectilinear path to a guide surface. A pen is secured to the chain and is slideably retained in a slot within the lower surface of the stylus arm. A potentiometer resistance element is suspended beneath the stylus arm and a wiper on the stylus arm contacts the potentiometer element to develop a position signal which is utilized to reduce stylus position error.
U.S. Pat. No. 4,180,823 to Guyot et al. discloses a control device for controlling the movements of two carriages of a duplex recorder. The device includes a flexible traction means connected to a first carriage and a drive means associated with the traction means for moving the first carriage selectively in two opposite directions.
U.S. Pat. No. 2,149,216 to Gravley discloses a motion-converting mechanism for magnifying and reducing linear motion and for converting linear motion into angular motion and vice versa.
None of the above references, however discloses a device which accomplishes the objects of the present invention.
Accordingly, it is an object of the present invention to provide an improved rectilinear pen movement apparatus for recording galvanic skin resistance.
This and other objects and advantages of the present invention will become more apparent in the following figures and detailed description.